Free platelet count and size distribution during C1q inhibition of collagen-induced platelet aggregation.

Electronic free platelet counting was more sensitive than turbidimetry to detect collagen-induced platelet activation in human platelet-rich plasma. Purified human C1q exhibited a greater inhibitory effect on collagen-induced platelet aggregation in turbidimetry than free platelet counting. Because the change from small to large platelet aggregates is responsible for the continuing increase in light transmission, C1q was likely more capable of blocking the formation of large platelet aggregates than the formation of small aggregates from single platelets. The rate of change by collagen in light transmission and free platelet count was reduced in the presence of C1q but the timing of the peak response remained the same. Electronic platelet sizing revealed that the volume of single platelets transiently increased during the turbidimetric "lag phase". The mean, mode and median volume of the remaining free platelets then decreased, suggesting a selective loss of large, functionally more active platelets and/or platelet degranulation. C1q had no effect on the volume increment during the "lag phase", but reduced the subsequent fall in the volume of free platelets.